The present invention relates to arc protection for power distribution lines and more particularly to structured arrangements employed to protect covered overhead power distribution lines against damage from fault current arcing.
Electric power distribution lines are normally classed as those which operate at 34 kV or less line to line, but usually no lower than 4 kV line to line. Overhead distribution conductors are insulated from ground using stand-off or string insulators on support poles. Adequate insulation from ground is achieved without covering the conductors with an insulating material. Bare distribution conductors are in use throughout the United States.
A significant percentage of installed power distribution lines are provided with conductors having an insulation covering which reduces hazards to life and property near or close to the lines. Covered conductors also provide certain other advantages over bare conductor circuits. For example, momentary tree contact is less likely to fault a covered conductor than a bare conductor. Momentary phase-to-phase contact caused by wind deflection will fault a bare conductor circuit, while a covered conductor circuit would not be affected under the same circumstances.
Covered distribution conductors can and often do create system maintenance problems as a result of conductor damage caused by lightning induced fault currents. Thus, experience has shown that covered conductors burn down more frequently than bare conductors. A fallen overhead phase conductor can cause a high impedance fault on distribution circuits, such as when a phase conductor falls, without contacting another phase or neutral conductor, and comes to rest on an asphalt or other high impedance surface. The resulting fault current magnitude is sometimes not sufficient to cause operation of the overcurrent protection equipment. In addition to interrupting customer service, the undetected live wire is a threat to public safety and a fire hazard.
Lightning may strike an overhead distribution conductor anywhere along its length and it can and often does arc to another conductor at a weak point. The most probable arcing occurs with common vertical lines with current flowing between the top phase conductor and the neutral conductor. Some problem exists for flashover from the top conductor to another phase conductor, sometimes involving all phases and the neutral.
Lightning can initiate power frequency fault current by ionizing a small path of gas between the conductors. This often occurs where the conductors have their insulation stripped back for necessary interconnections or attachment to support insulators.
The magnitude of the power frequency fault current is a function of the line voltage, the circuit impedance and other system parameters. Secondary functions such as arc bending winds and humidity also affect the fault current magnitude.
The fault current duration depends on the speed with which circuit interrupters function to open the faulted circuit. Conductor damage at the point of arcing varies in accordance with the conductor temperatures produced by fault current heat which depends in turn on the magnitude and duration of the fault current. Often, a single arc event is sufficient to melt enough conductor metal to cause the conductor to lose its needed tensile strength. It then falls to the ground as a result of a structural failure. In other cases, it may require two or three arc faults at the same point over a period of 20 or 30 years to produce a line failure. Failure can also occur some time after damage by normal load current heating because of the reduced current carrying capability resulting from the arc damage.
When an overhead conductor in a multi-grounded neutral distribution system breaks and falls to the ground without simultaneously contacting the multi-grounded neutral conductor, there is a significant probability of it coming to rest on a high-impedance surface, such as concrete, asphalt or dry earth. As previously indicated, the resulting fault current may not be sufficient to cause operation of the overcurrent protection equipment. The problem is further aggravated by the use of covered phase conductors which may increase the fault impedance and further reduce the fault current magnitude. In addition to interrupting customer service, the undetected live wire is a threat to public safety and a fire hazard.
Clearly, if reliability and safety advantages are to be gained from the use of covered distribution lines as opposed to uncovered lines, conductor arc damage needs to be avoided where conductors are stripped of insulation for interconnection or support. Thus, covered conductors need to be protected against burndown to be more reliable and safer than comparable bare conductor circuits.
The cross-referenced application of Shankle et al. (U.S. patent application Ser. No. 248,789, filed Mar. 30, 1981) discloses a prior art arc protection clamp of simple geometry and lacking several features and advantages included in the arc protection clamp of the present invention. For example, the prior art clamp lacks features for preventing weldment of the clamp members together by the arc heat, and lacks means to prevent splatter of molten metal from the clamp onto conductor insulation during an arc protection operation. These and other advantages of the present invention will be more completely discussed below in the "DESCRIPTION OF THE PREFERRED EMBODIMENTS".
Arrangements have been employed in the past to prevent corona on power lines. Corona does not normally damage conductor metal but it does produce television and radio interference.
A representative clamp type device for corona prevention is shown in U.S. Pat. No. 3,773,967 issued to R. Sturm. Another clamp type device is shown in U.S. Pat. No. 3,046,327, issued to R. Harmon. In the '327 patent, the clamp device is bridged across a portion of the bare conductor and an adjacent conductor portion strengthened with an armor covering.
Neither this art nor other known prior art is addressed to the need for protection against fault arc burndown of covered power conductors.